1. Field of the Invention
This invention relates generally to fabrication of the electronic packages, including single-chip packages and multichip packages, and in particular, to fabrication of a plurality of cavity down chip packages having heat spreaders which also function as stiffeners attached thereto.
2. Description of the Prior Art
With continuing advances in the semiconductor industry, electronic circuitry and electronic packaging are often designed to utilize as little space as is practicable. Circuit space often is a valuable asset, which needs to be conversed, and a miniaturization of electronic circuits often improves speed, reduces noise and leads to other performance advantages. Such a miniaturization is desirable in electronics employed in various applications, such as aircraft, automobiles, cellular phones, hand carry computers, hand carry camcorders, etc. However, heat density problems often arise with increased miniaturization, since the amount of heat generated by the device increases as the number of transistors fabricated onto a single semiconductor device increases.
One type of semiconductor chip package includes one or more semiconductor chips attached to a substrate, e.g. a ceramic substrate or a plastic substrate, wherein a ceramic substrate uses ceramic material as the insulating layers while a plastic substrate uses a plastics-based material as the insulating layers. Such a semiconductor chip package, conventionally termed a chip carrier, is generally interconnected on a printed circuit card or printed circuit board. Chips can be attached to the substrate in several ways. Currently, the most popular way is wire bonding, in which electrical connections are made by attaching very small wires from the device side of the chip to the appropriate points on the substrate. Another way of attachment uses small solder balls to both physically attach the chip and make required electrical connections, which is so called flip chip bonding.
Assorted methods have been employed to mount integrated circuit chips in plastic packages for a lower cost packaging means as compared with ceramic packages. While the plastic packages are further recognized to provide several important advantages for the chip operation as compared with ceramic packages, including higher current carrying capacity, lower dielectric constant for short operational delay times, along with reduced inductance and capacitance. However, low temperature stability experienced with plastic packages still remains as a problem. This issue has gained much attention in the development of modem plastic packages. One solution to this issue is to adopt a cavity down chip package structure, in which a heat slug or a heat spreader is attached to the bottom of the package and the chip is mounted in a recess cavity!with its open side facing toward the printed circuit card or printed circuit board.
FIG. 1 illustrates a typical cavity down chip carrier 100. It includes a plastic wiring substrate 101 with a recess cavity 102 and a heat slug or heat spreader 103 bonded to said substrate 101 with assistance of a bonding layer 104. A side-wall electrically and/or thermally conductive layer 105 may be made to connect the heat spreader with the wiring layer in said substrate 101 for further enhanced thermal or electrical performance. A chip 106 is mounted on the heat spreader 103 inside the recess cavity 102. The conductive wires 107 are used to interconnect the chip 106 to the substrate 101. After the wire bonding process, the cavity 102 is filled up with an encapsulant 108 to cover and protect the bonding wires 107 and chip 106 against environmental degradation. The external connection means 109, by which the substrate 101 is electrically connected to a printed circuit board 110, are attached to appropriate areas on the top surface of the substrate 101. The external connection means 108 may be conductive pins or solder balls or columns as utilized in plastic pin grid array (PPGA) or plastic ball grid array (PBGA) or plastic column grid array (PCGA). As an alternative structure, an additional heat sink may be attached to the backside of the heat spreader 103 if necessary for further enhancing heat dissipation.
In order to overcome warparge or twist in making a cavity down plastic chip carrier, a stiffener or more copper layers may sometimes be embedded. A typical example is shown in FIG. 2. A prior art cavity down plastic chip carrier, so called super ball grid array package, includes a circuit substrate 201 having a first surface 201a and a second surface 201b opposite the first surface and an internal copper layer 202 on the first surface 202a of which is attached to the second surface 201b of the circuit substrate 201 by means of an adhesive 203. On the respective central portions of the circuit substrate 201 and the copper layer 202 are formed an opening so that the opening forms a recess cavity 204 through the circuit substrate 201 and the copper layer 202. A heat spreader 205 is attached to the other surface 202b of the copper layer 202 with using an adhesive 206 for improving the heat dissipation characteristics. A chip 207 is mounted on the heat spreader 205 inside the recess cavity 204 with assistance of an adhesive 208. The conductive wires 209 are used to interconnect the chip 207 to the substrate 201. After the wire bonding process, the cavity 204 is filled up with an encapsulant 210 to cover and protect the bonding wires 209 and chip 207 against environmental degradation. The solder balls 211, by which the substrate 201 is electrically connected to :a printed circuit board 212, are attached to appropriate areas on the first surface 201a of the substrate 201. As an alternative structure, an additional heat sink may be attached to the second surface 205b of the heat spreader 205 if necessary for further enhancing heat dissipation.
U.S. Pat. No. 6,034,427 (J. J. D Lan et al.) teaches the use of a stiffener for a plastic cavity down BGA, in which the stiffener is first attached to a circuit board with assistance of a prepreg and then a heat spreader is attached to the stiffener by using an adhesive film. In this method, when the heat spreader is made of copper base material, e.g. copper or copper base alloy, which is well known a relatively soft material, the cure shrinkage of the adhesive film may cause warpage during heat pressing the heat spreader together with the circuit board containing the stiffener.
U.S. Pat. No. 6,060,778 (T. S. Jeong et al.) also teaches a similar method for making a plastic cavity down BGA, which states that the BGA package has excellent heat dissipation capability, a low weight, a thinner thickness and a low manufacturing cost. The method involves the use of a first thermally conductive layer, which is attached to a circuit substrate. A second thermally conductive layer (i.e. a heat spreader) is attached to the circuit substrate containing the first thermally conductive layer. However, the method is depicted in the FIG. 7 of the said patent but not included in the claims of the said patent. This method would encounter the same problem as described above for U.S. Pat. No. 6,034,427, i.e. warpage of the fabricated BGA is difficult to be eliminated.
It is therefore an object of the present invention to provide a method to make a plastic cavity down chip carrier having excellent heat dissipation capability, a low weight, absence of warpage and twist, and low manufacturing cost. Briefly, the present invention provides a method for first bonding two or more thermally conductive sheets to form a stiff heat spreader element and second bonding the stiff heat spreader element with a circuit board (or say integrated circuit substrate). This method is clearly different from the methods proposed by the U.S. Pat. Nos. 6,034,427 and 6,060,778, since both prior methods bond a first thermally conductive sheet with a circuit board and then bond a second thermally conductive sheet to the circuit board which already carries the; first thermally conductive sheet. There is one more clearly different point. That is, the method in the present invention proposes to use a prepreg or prepregs as the adhesive layer for bonding the thermally conductive sheets and a non-prepreg material as the adhesive layer for bonding a thermally conductive sheet with a circuit board, whereas the method propose by U.S. Pat. No. 6,034,427 uses a non-prepreg material as the adhesive layer for bonding the thermally conductive sheets and a prepreg for bonding a thermally conductive sheet with a circuit board. According to the present invention, the use of a prepreg for bonding the thermally conductive sheet can form a very stiff heat spreader element. After the stiff heat spreader element is attached to a circuit board, the stiff heat spreader element is able to function as a stiffener for the resultant cavity down chip carrier, so that warpage and twist of the resultant cavity down chip carrier could be greatly alleviated.
It is therefore an objective of the present invention to provide a stiff heat spreader element for making a cavity down plastic chip carrier having benefits of excellent heat dissipation characteristic, low weight, small thickness and low warpage and twist.
Another objective of the present invention is to adopt a first bonding sheet for bonding a first thermally conductive sheet and a second thermally conductive sheet to form a first stiff heat spreader element. The first bonding sheet with any configuration is a prepreg or prepregs based on a fiber-reinforced resin. The second thermally conductive sheet has an opening for receiving an electronic chip and the electronic chip can be directly attached onto the surface of the first thermally conductive sheet.
Another further objective of the present invention is to adopt a second bonding sheet for bonding a plastic circuit board having an opening for receiving an electronic chip and the first stiff heat spreader element. The second bonding sheet with any configuration is made of a single adhesive layer or a stack of more adhesive layers. The adhesive layer is made of an adhesive material, a flake-filled adhesive material, a fiber-filled material, or a particle-filled material. The adhesive layer is not a prepreg.
Another further objective of the present invention is the provision of a second stiff heat spreader element formed by stacking a first thermally conductive sheet and two or more second thermally conductive sheet with the use of the first bonding sheet is between the thermally conductive sheets. The first bonding sheet with any configuration is a prepreg or prepregs based on a fiber-reinforced resin. The second thermally conductive sheets having openings for receiving an electronic chip and the electronic chip can be directly attached onto the surface of the first thermally conductive sheet.
Another further objective of the present invention is to adopt a second bonding sheet for bonding a plastic circuit board having an openings for receiving an electronic chip and the second heat spreader element. The second bonding sheet with any configuration is made of a single adhesive layer or a stack of more adhesive layers. The adhesive layer is made of an adhesive material, a flake-filled adhesive material, a fiber-filled adhesive material, or a particle-filled adhesive material. The adhesion layer is not a prepreg or prepregs.